Display system with pixel electrodes separated from a common electrode by different optical distances

ABSTRACT

A display system comprises an electro-optic layer having first and second opposite major surfaces, a first electrode proximate the first surface of the electro-optic layer, and an electrode array. The electrode array includes a first plurality of pixel electrodes operatively coupled to the electro-optic layer proximate it&#39;s second surface and is separated from the first electrode by a first optical path length. The electrode array also includes a second plurality of pixel electrodes operatively coupled to the electro-optic layer proximate it&#39;s second surface and is separated from the first electrode by a second optical path length. The first and second pluralities of pixel electrodes form an irregular pattern.

CROSS-REFERENCE TO RELATED APPLICATIONS

This is a continuation-in-part of U.S. patent application Ser. No.09/797,540 filed on Feb. 28, 2001 now U.S. Pat. No. 6,636,287 andentitled Display System With Pixel Electrodes.

FIELD OF THE INVENTION

The present invention relates to the field of display devices such as aliquid crystal display device, and more particularly to a display systemwherein pixel electrodes having a first distance from a common electrodeand pixel electrodes having a second distance from the common electrodeform an irregular pattern.

BACKGROUND OF THE INVENTION

Liquid crystal displays can be either transmissive or reflective. Theirbasic construction consists of a liquid crystal material, which is aform of an electro-optic layer, between two substrate plates which haveconducting electrodes on their inner surfaces. At least one of theelectrodes is a transparent electrode, consisting of a high refractiveindex material such as indium tin oxide (ITO) placed on top of atransparent substrate such as plastic or glass, with a lower refractiveindex. Other layers of lower refractive index, such as a passivationlayer and a polyimide alignment layer, may be placed on top of the ITOelectrode. Because of the refractive index mismatch between thesematerials, a certain amount of reflection occurs at the interfacesbetween the liquid crystal and the ITO layer and between the ITO layerand the substrate material. This results in multiple reflectionsoccurring inside the liquid crystal cell which can constructively ordestructively interfere depending on the cell gap and the wavelength ofthe light. This causes highly visible and undesirable coloredinterference fringes appearing on the display when the cell gap isnon-uniform, especially when the cell gap is relatively thin and theillumination spectrum consists of one or more narrow band peaks.

Liquid crystal displays, and in particular liquid crystal on silicondisplays, can suffer from problems if the liquid crystal cell gap ismade non-uniform during construction. Alternatively, the gap may beconstructed uniformly but can subsequently be subjected to stress thatcan cause a distortion such that the cell gap is not uniform over thearea of the liquid crystal display. Typically the liquid crystal layeris only a few microns thick, which is a distance scale that can resultin optical interference patterns being formed with many light sources,including LED illumination. In a reflective liquid crystal cell, wherethe effect of non-uniformity is doubled, a change of the order of 0.2microns is enough to cause an interference fringe. Indeed, this problemcould be more serious than any other visible effect of the underlyingcell gap non-uniformity, and so this phenomenon can result in a highreject rate. The fringes can be eliminated by making the cell gapextremely uniform, but this is difficult to achieve with a high yieldusing present day manufacturing techniques.

Fringes occur because optical interference inside the cell (sometimesenhanced by a polarization effect) changes the amount of light reflectedfrom the display. This interference is a function of cell gap, sochanges in cell gap (that would not otherwise be enough to causeproblems in other ways) show up as changes in brightness. In certaindisplays, this change will be much worse for certain colors and sofringes may only be seen in images having those colors, such as redimages. FIG. 1 is an illustration of this effect. The peaks on the curve12 are separated by about 0.2 microns in cell gap, and the underlyingintensity change as a function of thickness is small enough that asmooth variation of that amount would not typically be a problem. Thegraph 10 shows the red intensity as a function of the cell gap. Dot 14on the curve 12 represents the red intensity for a particular pixel andthe dot 16 represents the red intensity of a nearby pixel which wouldotherwise display the same red intensity as the pixel represented by thedot 14 except that the cell gap for this pixel differs from the cell gapof the pixel represented by dot 14. If these pixels are reasonably closetogether, then this is typically seen as an objectionable fringe. Thefringes give a contour map of the cell gap, with the transition of lightto dark representing 0.1 microns and a full fringe dark-light-darkrepresenting 0.2 microns. Clearly it would be advantageous if thesevariations of the cell gap did not cause such visual artifacts.

Interference fringes, in general, are reduced by suppressing at leastone of the reflections that are required to form two interfering beams.In a reflective display, the only component that can be suppressed isthe reflective beam component from inside of the glass cover, where thetransparent conductive electrode is located. Multi-layer coatingtechniques provide one way to reduce interference fringes. U.S. Pat. No.5,570,213 describes a way to reduce the interference fringes by addingadditional layers on either side of the ITO layer. These additionallayers act as a broadband anti reflection coating which effectivelyrefractive index matches the ITO layer to the substrate material on oneside and the liquid crystal on the other side. While these layers willdecrease the intensity of the observed interference fringes, they arenot completely satisfactory because, being a birefringement material,the liquid crystal has two principal refractive index values and it isnot possible to simultaneously index match to both of these indices oversufficiently broad spectral range. Furthermore, these anti reflectioncoatings can contain up to 20 different dielectric layers which can bequite expensive to manufacture.

A different approach to eliminate the colored fringes caused by cell gapvariations is taken by U.S. Pat. No. 4,693,559. In this case thesubstrate is roughened with a plurality of depressions, prepared byetching or embossing. The thickness variation within each depressionproduces a color variation of substantially the entire color spectrumwhich the eye averages out to a neutral additive color mix since thedepressions are relatively small in size. While this method is effectiveat eliminating fringes, it does introduce a considerable amount of lightscattering due to the roughened surface. This roughened surface is theITO layer. This would make this method unsatisfactory to use in opticalconfigurations where light loss due to scattering cannot be tolerated,such as in projection applications.

A related approach cited in U.S. Pat. No. 5,418,635 adds a plurality ofconvex portions of two or more different heights formed from photoresistbumps and then covers them with a polymer resin film to give the surfacea continuous wave shape without any flat portions. Because there are noflat portions between the top and bottom of the liquid crystal layer,the multiple reflections causing the interference colors cannot occur.While this method has been demonstrated to be effective in reducinginterference colors, it suffers from the same scattering limitation ofthe previous example with the roughened surface.

U.S. Pat. No. 4,632,514 provides a different cell gap under the red,green and blue color filters which is proportional to the dominantwavelength of each of the filters. Thus each separate pixel for thesethree colors has a different cell gap. In one example from this patent,a 5.4 micron gap is provided under the red filter, a 4.8 micron gapunder the green filter, and a 4.0 micron gap under the blue filter.Multiple cell gap color displays provide an improved contrast andviewing angle compared with color displays only having a single cellgap. In this prior patent, the cell gaps themselves are designed to beproportional to the dominant wavelength of each of the filters and thusthis design is limited to use in a color display and would not be usefulin color displays where colors are generated by other methods, such astime sequential color. Similarly, this prior approach is not useful formonochrome panels that are used in “3-panel” systems where color isgenerated with 3 monochrome panels, each illuminated with a differentcolor.

SUMMARY OF THE INVENTION

According to an aspect of the invention, there is provided a displaysystem comprising an electro-optic layer having first and secondopposite major surfaces, a first electrode proximate the first surfaceof the electro-optic layer, and an electrode array. The electrode arrayincludes a first plurality of pixel electrodes operatively coupled tothe electro-optic layer proximate the second surface and is separatedfrom the first electrode by a first optical path length. The electrodearray also includes a second plurality of pixel electrodes operativelycoupled to the electro-optic layer proximate its second surface andseparated from the first electrode by a second optical path length. Thefirst and second pluralities of pixel electrodes form an irregularpattern.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a graph which indicates the modulation of light due tointerference of the intensity reflected from a liquid crystal cell as afunction of cell gap.

FIG. 2 is a graph showing the modulation due to interference fringeswith various marks to illustrate that two different paths lengths can bespatially averaged together to reduce the effect of this modulation.

FIG. 3A shows a cross-sectional view of one pixel of one embodiment of adisplay system of the present invention.

FIG. 3B shows a cross-sectional view of two adjacent pixels, where thepixel electrodes are similar to the type shown in FIG. 3A.

FIG. 4 is an alternative embodiment of the display system shown in FIG.3A in which the pixel electrode is stepped, and then planarized toprovide a substantially flat surface for the LC, and hence asubstantially uniform cell gap.

FIG. 5 shows another alternative embodiment of a display system of thepresent invention.

FIG. 6 shows an alternative display system of the present invention inwhich a pixel electrode is transparent and is stepped to providedifferent distances relative to a common transparent electrode, such asan ITO layer on the top substrate.

FIG. 7 shows a plot of transmittance of a 90° twisted nematic liquidcrystal cell with red LED illumination. The curves are for two pathlengths, and the resulting spatial average of these two path lengths.

FIG. 8 is a plot as in FIG. 7 except in green illumination.

FIG. 9 is a plot as in FIG. 7 except in blue illumination.

FIG. 10 is a plot of the reflectance of a 45° twisted nematic liquidcrystal cell with red illumination. In this case, the different pathlengths are created with 0.1 microns of glass.

FIG. 11 shows the example shown in FIG. 10 except with greenillumination.

FIG. 12 shows the example shown in FIG. 10 except with blueillumination.

FIG. 13 shows an example pattern of path length differences which may beachieved by having pixel electrodes of different heights relative to acover glass electrode such as an ITO layer.

FIG. 14 shows another example pattern of path length differences.

FIG. 15 shows another example pattern of path length differences.

FIG. 16 shows another example pattern of path length differences.

FIG. 17 shows a more elaborate example pattern of path lengthdifferences.

FIG. 18 shows an example pattern of path length differences where thepath length differences are designed to be approximately an odd multipleof one-quarter of the wavelength of the light illuminating the pixels.

FIG. 19 shows another example pattern of path length difference whereeach pixel has four regions for producing path length differences for asingle pixel.

FIG. 20 shows an example pattern of path length differences where entirepixels are set to one path length or the other, and the distribution ofthese path lengths is arranged irregularly over the pixel electrodearray.

FIG. 21 shows an example pattern of path length differences where entirepixels are set to one path length or the other, and the distribution ofthese path lengths is arranged irregularly in a pixel electrodesub-array which is then repeated over the pixel array.

DETAILED DESCRIPTION

The subject invention will be described with reference to numerousdetails set forth below, and the accompanying drawings will illustratethe invention. The following description and drawings are illustrativeof the invention and are not to be construed as limiting the invention.Numerous specific details are described to provide a thoroughunderstanding of the present invention. However, in certain instances,well known or conventional details are not described in order to notunnecessarily obscure the present invention in detail.

The present invention can be used in many different types of displaysincluding both passive and active matrix type liquid crystal displays.The present invention is particularly useful with liquid crystaldisplays formed on integrated circuit substrates with reflective pixelmirrors. These types of displays are referred to as LCOS displays(liquid crystal on silicon). Various examples of such displays are knownin the prior art. For example, U.S. Pat. No. 6,078,303 describes indetail circuits and methods for display systems, and particularlycircuits and methods for display systems which drive a LCOS display witha time sequential color method. U.S. Pat. No. 5,426,526 also describesthe various circuitry in the integrated circuit which includes the pixelelectrodes which act as pixel mirrors in this type of display.Typically, LCOS displays are reflective displays and they may be microdisplays which are designed to create a small image on the integratedcircuit which is then magnified. The present invention may be used withsuch displays. Alternatively, the present invention may be used withtransmissive displays and with other types of electro-optic materialother than liquid crystals. The present invention may be used withmonochrome displays, time-sequential color displays, or spatial colordisplays. The above referenced U.S. Patents (U.S. Pat. Nos. 5,426,526and 6,078,303) are hereby incorporated herein by reference.

FIG. 2 illustrates conceptually how certain embodiments of the presentinvention function to reduce the fringe interference. In certainembodiments, each pixel is constructed to sample two points on the curveof intensity versus cell gap, such as the curve 20 shown in graph 18 ofFIG. 2. Thus, for example, a first pixel may sample the curve 20 atpoints 21 and 24, while a second pixel may sample the curve at points 22and 25 while a third pixel samples the curve at points 23 and 26. Foreach of these pixels, these two samples can be just averaged together(by spatial averaging) and thus this FIG. shows that the fringeinterference is largely suppressed by this technique. Thus, in certainembodiments, at least two different optical path lengths are distributedover the display in closely adjacent regions, such as the same pixel. Incertain embodiments the path length difference is chosen so that amaximum in the interference along one optical path length is a minimuminterference along another path length. By placing these two pathlengths in close proximity on a display, the eye will only respond tothe average of these two intensities which remains substantiallyconstant regardless of cell gap variations. In one embodiment, twooptical paths are provided for each pixel in a reflective display bydividing each pixel into two regions of substantially equal area byproviding a reflector under the pixel having two different levels. Thisarrangement results in two different cell gaps within the pixel. Toeffectively average out the fringes, the difference in the cell gap incertain embodiments should be approximately an odd multiple of a quarterwavelength of light within the liquid crystal material. Thus a cell gapdifference of 0.1 microns, 0.3 microns, or 0.5 microns would beeffective, but 0.1 microns is preferred because it provides fringesuppression over a wider range of wavelengths and makes it easier tomanufacture the display.

FIG. 3A shows one embodiment of a display system of the presentinvention. In this cross-sectional view of FIG. 3A, the display device30 includes a liquid crystal layer 36 which is sandwiched between an ITOlayer 38 and an optional passivation layer 34. The ITO layer 38 has beenapplied to a cover glass 39. The passivation layer 34 is disposed abovea pixel electrode 32 which also acts as a reflector. Thus, the display30 represents an example of a liquid crystal display device which is areflective device. The aluminum reflector is also the pixel electrodewhich controls the liquid crystal in order to produce a particularoptical state for the pixel. It will be understood that in the case of aLCOS display that the pixel electrode 32 will be disposed above asemiconductor substrate, such as a silicon integrated circuit whichincludes the various circuitry used to control each pixel electrode. Asnoted above, U.S. Pat. Nos. 6,078,303 and 5,426,526 describe variousexamples of LCOS display devices and the circuitry used in those displaydevices for controlling pixel electrodes. It can be seen from FIG. 3Athat each pixel electrode has a stepped surface such that there are twoupper surfaces of the pixel electrode and thus two different distancesbetween the surface of the pixel electrode and the opposing electrode,which in this case is the ITO layer 38. If the step is made such thatthe difference in height between the two surfaces of the pixel electrodeis approximately 0.1 microns, then, as noted above, this will tend tocancel or suppress the fringe modulation. In the example of FIG. 3A, thepath length change is achieved in this case simply by achieving thechange in the liquid crystal cell gap, which is the gap between the ITOlayer 38 and the upper surface of the aluminum reflector, which is thepixel electrode 32.

The stepped pixel electrode 32 may be fabricated by changing thethickness of a portion of each pixel electrode (e.g. by applying a maskwhich partially covers each pixel electrode and by etching away theexposed portion of the pixel electrode or by depositing the pixelelectrode on an underlying substrate which has been processed to havedifferent levels). Typically, a passivation layer would be applied ontop of the pixel electrode, such as passivation layer 34, and then thisis typically followed by the alignment layer such as a polyimide, as iswell known in the art. As noted above, the pixel electrode may bedeposited on an underlying substrate which has been processed to havedifferent levels. One method for performing this is to etch thedielectric which is to receive the metal pixel electrode to providesteps in the dielectric. This may be accomplished by masking portions ofthe dielectric with a photoresist and by etching the exposed portions ofthe dielectric while the other portions are protected by the photoresistmask. The photoresist mask can then be removed and then the metaldeposited into the stepped substrate. Following the deposition of themetal pixel electrodes, a spin-on glass is applied to fill the gapsbetween pixel electrodes and then an etch back is performed which stopsat the detection of the metal pixel electrodes and then a finalpassivation layer is applied. Either method may be used to produce thestructure shown for the particular pixel electrode 32 shown in FIG. 3A.FIG. 3B shows another cross-sectional view of a display 40 which issimilar to the display 30 except that two adjacent pixel electrodes 33and 34 are shown on the upper surface of the substrate 42 which may bean integrated circuit such as the integrated circuits described in U.S.Pat. No. 6,078,303 or U.S. Pat. No. 5,426,526. Thus it will beappreciated that interconnections to the pixel electrodes 43 and 44,while not shown, do exist to the pixel electrodes in order to controlthe liquid crystal layer 47 to provide optical states for each of thesetwo pixels. The pixel electrodes 43 and 44 each include two pixelelectrode surfaces which have different heights or distances relative tothe ITO transparent electrode 49 which is attached to the cover glass50. Thus two different optical lengths exist for each pixel. As shown inFIG. 3B, the pixel electrode 43 includes a first pixel electrode surface43A and a second pixel electrode surface 43B. Similarly, pixel electrode44 includes a first pixel electrode surface 44 a and a second pixelelectrode surface 44 b. These surfaces are covered by a passivationlayer 45 which is similar to the passivation layer 44 of FIG. 3A.

FIG. 4 shows another example of a display system according to oneembodiment. FIG. 4 shows a stepped pixel electrode 78 which represents asingle pixel electrode on a substrate 76. It will be appreciated thatthere are typically many such stepped pixel electrodes on the substrate76 so that the display includes many pixels which can be used to form amultipixelated image. Similarly, it will be appreciated that FIGS. 5 and6, while showing a single pixel electrode, also can be considered todepict a portion of a display system which includes multiple such pixelelectrodes disposed on a plane over a substrate, such as an integratedcircuit as in the case of a LCOS display device in which the substrateis an integrated circuit containing the circuitry which drives thevarious pixel electrodes. Returning to FIG. 4, it can be seen that thestepped pixel electrode 78 has a first pixel electrode surface which iscloser to an opposing electrode, which may be an ITO layer 82, than asecond pixel electrode surface. In particular, pixel electrode surface78A is closer to the ITO layer 82 than pixel electrode surface 78B. Adielectric layer 86 which may be a spin-on glass (SOG) is disposed abovethe pixel electrode surface 78B so that the resulting top surface of thesubstrate 76 is substantially flat. This allows the passivation layer 88to also be substantially flat, which improves the performance of aliquid crystal display in many cases. Light traveling through the twodifferent regions defined by the two pixel electrode surfaces will havetwo different optical path lengths. That is, light traveling in thevicinity of the pixel electrode surface 78A will have a shorter opticalpath length than light traveling toward and then away from the pixelelectrode surface 78B.

One method for fabricating the display device 75 shown in FIG. 4 willnow be described. Conventional semiconductor fabrication processing maybe utilized to create the circuits in an integrated circuit which willserve as the substrate for a LCOS display in the substrate 76. Prior todepositing the metal pixel electrode layers, the underlying dielectricwhich serves as the surface for the pixel electrodes is step etched inorder to create steps in this dielectric which is typically anintermetal dielectric. A technique for creating this step etch has beendescribed above. Then the aluminum pixel electrode material is depositedonto the intermetal dielectric which has been etched to create the stepsnecessary for each pixel electrode. Then a spin-on glass is applied, andan etch back of the spin-on glass is performed, and this etch back stopswhen the metal “signature” of the high portions of the pixel electrodemetal, such as pixel electrode surface 78A, are detected and thisstopping of the etch back leaves some spin-on glass on top of the lowerpixel electrode surfaces, such as pixel electrode surface 78B. Then apassivation deposition is done to apply the passivation layer 88, whichcould, for example, be a thin layer of SiO2. Next, an ITO layer 82 isapplied to a glass substrate 84 to create a counter electrode on theglass substrate 84. Spacers are applied around the periphery of thedisplay device in order to define the space between the passivationlayer 88 and the ITO layer 82. Then a liquid crystal material 80 isinjected into the space between the passivation layer 88 and the ITOlayer 82. It will be appreciated that in many embodiments, aconventional liquid crystal alignment layer may be applied to both thepassivation surface 88 and the ITO layer 82 in order to cause the liquidcrystal to align in a desired state, as is well known in the art.

FIG. 5 shows another embodiment of a display system according to anotherembodiment of the present invention. Display system 100 includes asubstrate 101 which may be an integrated circuit in the case of an LCOSdisplay device. A typical LCOS display device would normally use asingle crystal silicon substrate to fabricate the integrated circuitstherein as is well known in the art. The integrated circuitry, as isknown in the art, is coupled to each of the various pixel electrodes onthe upper surface of the substrate. A single pixel electrode 103 isshown in FIG. 5 but it will be appreciated that there are typically manysuch pixel electrodes in a plane on the upper surface of the substrate101. Each pixel electrode surface includes a first surface 103A and asecond surface 103B. In this case, the pixel electrode 103 issubstantially flat at its upper surface. Thus, this pixel electrodeappears to be a conventional pixel electrode surface which is found onconventional LCOS display devices. However, a portion of the pixelelectrode is covered by a dielectric 116 so that an optical path lengthinto and back from the aluminum reflector surface of the pixel electrode103 is different depending upon the particular surface that the lightstrikes. This can be seen by examining the optical path of light 112compared to the optical path of the light 114 as shown in FIG. 5. Inparticular, for a reflective display, light enters at path 112 andpasses through the glass substrate 111 and through the ITO layer 109 andthrough the liquid crystal 107 and through the passivation 104 andstrikes the surface 103A and is reflected back through layers 104, 107,109, and 111. This optical path is different than the optical path takenby light path 114 because of the optical nature of the dielectric 116which is disposed over the pixel electrode surface 103B. Typically, thisdielectric 116 has a significantly different refractive index from theliquid crystal's refractive index in order to provide a sufficientlydifferent path length associated with the two different pixel electrodesurfaces 103 a and 103 b. If the difference between the refractive indexof the dielectric 116 and the refractive index for the liquid crystal107 is small, then the thickness of the dielectric 116 can be increasedto accumulate the optical path difference. Of course each of the pixelelectrode surfaces 103A and 103B can be coated with the dialectic 116 aslong as the thicknesses of the two dielectric coatings are sufficientlydifferent to achieve the desired optical path length difference. In somecases it may be advantageous to place the dielectric 116 on top of theITO electrode 109 of the upper glass substrate 111 instead of the pixelelectrode 103 of lower substrate 101. The effect of reducing fringeswill be the same but under some circumstances this alternate design maybe easier to fabricate.

FIG. 6 shows another embodiment of a display system according to thepresent invention. Display device 125 may either be a transmissivedisplay (if the layer 129 is transparent) or it may be a reflectivedisplay device if the layer 129 is reflective. The display device 125includes a glass substrate 135 and a transparent electrode layer 133,which may be an ITO layer. A liquid crystal layer 131 is disposedbetween the ITO layer 133 and the passivation layer 139. As noted above,the ITO layer 133 and the passivation layer 139 will typically includean alignment layer in cases where the liquid crystal 131 requires analignment for its display properties. A transparent electrode, such asanother ITO layer 137, is disposed between the passivation layer 139 andthe layer 129 which includes two surfaces 129 a and 129 b which havedifferent distances relative to the ITO layer 133. In the case of areflective display device, the layer 129 is a stepped mirror for eachpixel, and there will typically be a dielectric layer which isolates thetransparent electrode layer 137 from the reflective mirror layer 129 ifthe layer 129 is a mirror. In an alternative embodiment, the layer 129may be a transparent material which is not conductive and which merelyprovides the stepped surface for the transparent electrode layer 137.The substrate 127 may be a thin film transistor substrate such as thoseused in conventional notebook computers with liquid crystal displays(such as the Macintosh PowerBook G3 computer from Apple Computer, Inc.).An alternative embodiment of the display device 125 may have the samestructure as shown in FIG. 6 except that an extra passivation layer isapplied above the stepped surface 129 b, above the transparent electrode137 so that the underlying surface on which the passivation layer 139rests is substantially flat. This is similar to the embodiment shown inFIG. 4 in which the dielectric 86 creates a substantially flat surfaceover the entire pixel electrode 78, and thus the passivation layer 88 issubstantially flat.

A method for choosing the magnitude of the optical path lengthdifference will now be described. The optical response of a display cellconfiguration as a function of cell gap, at the wavelength of interest,is calculated or measured at cell gaps surrounding the cell design.Calculations may be performed with commercial liquid crystal modelingsoftware such as DIMOS (from Autronic Melchers). From these measurementsor calculations, one can determine the change in optical path that movesa peak into a trough. This change is typically the desired optical pathlength change. In many situations, this will simply result in a quarterwave of optical path length difference. For example, if the wavelengthof interest is red light at 630 nanometers (nm), and the refractiveindex of the medium in which the extra optical path length is formed is1.5, then the resulting thickness of this piece of material is 630nm÷(4×1.5)=105 nm. If the path length changes are formed by extradistance in the birefringement liquid crystal media (such as in thedisplay system shown in FIG. 3A), then this is, in principle, differentfrom forming the extra path length in an isotropic material such as aspin-on glass (such as the display system shown in FIG. 4). In practice,the differences are usually small enough to be neglected because the SOGand liquid crystal have similar refractive indices, but eitherfabrication technique can be exactly accounted for in the modelingsoftware.

There are also solutions that result from moving a peak of the curveinto a trough that is not immediately adjacent to it. These solutionsoccur close to odd multiples of a quarter wavelength of the light (e.g.three times a quarter of a wavelength or five times a quarter of awavelength, etc.). The disadvantage of these higher order solutions isthat their effect diminishes more rapidly with a change in wavelengththan a solution which is at approximately one quarter of a wavelength.They are worth considering if, for example, there is a convenientfabrication technique easily available to make path length differencesclose to one of these solutions. Another reason for considering higherorder solutions is that they can, in principle, be used to providefringe canceling at more than one wavelength. For example, a path lengthdifference of 7 times one quarter of a wavelength at 450 nm is also apath length difference of 5 times a quarter wavelength at 630 nm. Ateven higher orders, similar phenomena occur (a path length difference of13 times one quarter of a wavelength of 450 nm is also 11 times onequarter of a wavelength at 532 nm, and 9 times one quarter of awavelength at 650 nm). Hence, it is possible to cancel fringes atmultiple wavelengths simultaneously with only two path lengths.

An analysis shows that in general the amplitude of the modulation causedby interference is itself a function of cell gap. One example of this iswhere the amplitude of the fringes decreases with increasing cell gap,due to the coherence of the light source. If this effect is consideredsignificant, it is possible to apply a correction by biasing therelative areas of the pixel regions. In this example where the fringeamplitude decreases with increasing gap, the correction would be toslightly increase the area of the pixel which has the longer opticalpath length. By doing this, one can achieve better cancellation becausethe effect of less modulation over the larger area better balances theeffect of the larger modulation over the smaller area, when the pixelintegrated by the human eye. Clearly, this technique can be used tospatially integrate contributions from more than two optical paths. Thismay be analyzed using the same method as above, by adding the curvesfrom multiple contributions. This could be done for situations in whichthe suppression of fringes over a wider wavelength range is desired.

An example of this method of determining the optical path length ispresented in FIGS. 7, 8 and 9. In this example, a 90° twisted nematictransmissive cell is modeled in the commercial liquid crystal modelingsoftware known as DIMOS. FIG. 7 shows the transmittance of the lightfrom a particular red LED illuminator as a function of cell gap. Thethree curves 201, 203 and 205 show the transmittance of the cell, thetransmittance of the cell with an added 0.2 micron of path, and theresult of the spatial averaging of these two contributions. FIG. 8 showsin its graph 225 the same cell modeled with the light from a particulargreen LED and shows three curves of the transmittance of the cell, whichinclude curve 227 that shows the transmittance of the cell, curve 229which shows the transmittance of the cell with an added 0.2 micron ofpath, and the resulting curve 231 which is the average of these twocurves. It can be seen that the modulation is less to begin with, andthere is also a significant improvement even though the peaks andtroughs are not aligned perfectly (because the extra path length in thisexample was chosen to match the wavelength of the red LED). FIG. 9 showsthe same cell modeled in light from the blue LED and includes curves252, 254 and the spatial average curve 256 which averages the curves 252and 254.

A second example is presented in FIGS. 10, 11, and 12. In this example,a 45° twisted nematic reflective cell is modeled in DIMOS using the samethree LED spectra as the previous example. In this case, the opticalpath length difference is created with a 0.1 micron thick layer of aglass-like material with a refractive index of 1.5 and a dielectricconstant of 5.0. The liquid crystal cell gap is the same on both regionsof the cell, corresponding to the situation shown in FIG. 4. Thisparticular path length difference results in a dramatic reduction offringe modulation in both red and green as can be seen from FIGS. 10 and11, although the choice is biased toward the red, as the modulation fromthe fringe effect is larger for that color.

There are various different ways to arrange to have different opticalpath lengths over an area of pixel electrodes. Typically, it is expectedthat approximately equal areas of two different optical path lengths,which are spaced closely enough to be spatially integrated by the humanvisual system, will be preferred. While it is quite likely to mean thatone will desire to have one area of each path length in each pixel, thatmay not be required in high resolution displays that are not magnifiedvery much, and conversely, if the pixels are magnified to a largeextent, it may be better to have more than one area of each path lengthon each pixel. Examples of various possible layouts of the differentregions of optical path are shown in FIGS. 13-19.

FIG. 13 shows one exemplary pattern of path length differences. Thepattern 400 is an 8×4 array of pixels, where each pixel has one area ofeach of the two different path lengths. The different path lengths areillustrated by the white and gray regions. For example, pixels 401, 403and 405 each have two regions of different optical path lengths. Thesetwo different optical path lengths may be created in any of the varietyof ways shown above. For example, these path lengths may result from adisplay having a structure such as that shown in FIG. 5 or a structureshown in FIG. 4 or a structure shown in FIG. 3A or a structure shown inFIG. 6.

FIG. 14 shows another exemplary pattern of path length differences. Inthis arrangement of an array of 8×4 pixels, the left and right side ofthe pixels are of different optical path lengths, but are exchanged fromrow to row. In this arrangement, it may be expected that the slightluminance difference of the different sides of the pixels will spatiallyintegrate a little better than the example shown in FIG. 13. Theexchanging from row to row in the pattern 415 can be seen by comparingpixel 417 to pixel 421 in that the gray region is to the right for pixel417 and to the left for pixel 421.

FIG. 15 shows another exemplary pattern 430 of path length differences.In this arrangement, a central region of the pixel has a differentoptical path length from the edge region of the pixel. This can be seenby observing the gray regions within pixels 431, 433, and 435 relativeto the edges of each of these pixels. The pattern 430 is a 7×4 array ofpixels. The pattern shown in FIG. 15 has the advantage that the edge ofthe pixel mirrors, in those embodiments in which the height of the pixelmirrors is varied, are all at the same level, thereby simplifyingfabrication.

FIG. 16 shows another exemplary pattern 450 of path length differences.In this array of 7×4 pixels shown in the pattern 450, the differentoptical path length regions are arranged with a diagonal boundary foreach pixel. This can be seen in pixels 451, 453 and 455. This may beadvantageous if the liquid crystal is aligned diagonally, and if themechanism for forming the different optical path lengths results in somesurface unevenness. The liquid crystal will experience less disruptionif it is aligned parallel to this edge.

FIG. 17 shows another exemplary pattern 470 of path length differences.The pattern 470 shows an array of 8×4 pixels, including pixels 471, 473,and 475. In this arrangement, the different optical path length regionsare again arranged with a diagonal boundary. In this case, there areonly diagonal boundaries between the areas of different path lengths.Again, this may be advantageous if the liquid crystal is aligneddiagonally, and if the mechanism for forming the different optical pathlength results in some surface unevenness. Typically, it is desirablethat each pixel have approximately equal areas of each of the twooptical path lengths on each pixel. In the arrangement shown in FIG. 17,each pixel has two regions of one pathlength and one region of another.

FIG. 18 shows a particular pattern 500 which includes an array of 8×4pixels including pixels 501, 503, and 505. The pattern 500 represents asituation in which entire pixels are arranged to be alternatively ofdifferent optical path length. They are shown in a checkerboardarrangement, but alternatives, including vertical or horizontal stripes,could also be utilized. This could be done in situations where themagnification is low enough that the relatively low fringe inducedmodulation on individual pixels is not perceptible. This could also beextended to larger groups of pixels. An example of a pattern of largergroups of pixels is a 2 by 2 square of pixels set to one path length,and a neighboring square of 2 by 2 pixels set to the other path length.These 2 by 2 squares of pixels could be interspersed in, for example, acheckerboard pattern. This approach of setting entire pixels to one pathlength or the other may benefit optical efficiency because there is noregion inside the pixel that is undergoing a step in path length. Thisstep region can scatter some light out of the optical system and causesome loss of efficiency.

If entire pixels are arranged to contain a single path length, there isa possibility that certain displayed images may interact with thepattern of path-lengths in such a way as to cause fringes to becomevisible; e.g. if a fine checkerboard pattern was displayed on a pixelarray that had a corresponding fine checkerboard pattern of pixelpath-length differences. In this example, the different gray levelsbeing displayed as the two colors of the checkerboard might besufficiently different in their electro-optic responses to path-lengthchanges that the cancellation phenomenon might break down. An example ofthis break down could be where the image is made of alternating blue andred pixels arranged in a checkerboard pattern, and the pixels ofdifferent path-lengths are also arranged in a matching checkerboardpattern. It is possible that the red pixels could have a larger fringemodulation, as a function of cell gap, than the blue pixels and so theintensity variations seen on the red pixels would not be properlycancelled by the compensating variations on the blue pixels.Furthermore, even if the fringe modulations were similar so theintensities did cancel well, this example shows that there would be afringe induced color shift. This is because where the overall cell gapcauses the pixels that are red to be brighter; they are compensated byintermingled dimmer blue pixels. Similarly, where the overall cell gapis different and the blue pixels are brighter, they are compensated bydimmer red pixels. In this example the fringe pattern would cause acolor shift. This behavior is caused by the correlation between theapplied image data, and the underlying pixel path-length pattern. Thesolution to this problem is to choose an irregular pattern fordetermining which pixel is assigned which path-length in the pixelarray., The irregular pattern is much less likely to be correlated withthe image data, and so is much less likely to be prone to interactionswith image data. It is, of course, possible to find a particularpathological image for any given pattern of pixel path lengths but thepoint is that this is very unlikely to occur in the normal course ofevents.

The irregular pattern can be generated either by making a cell with anirregular distribution of the path lengths and replicating it, or bymaking the entire array with such an irregular distribution. These twoapproaches are illustrated in FIGS. 20 and 21. For example, FIG. 20shows an irregular pattern of pixels 530 with different path lengths.FIG. 21 illustrates an array of pixels 532 comprised of a plurality of3×4 irregular pixel sub-arrays. For example, the 3×4 irregular pixelsub-array comprised of pixels 534 is repeated several times throughoutpixel array 530. The irregular pattern can be chosen manually or bycomputer. In either approach, care should be taken that approximatelyequal quantities of each path length are use, and that no significantareas with too large a number of one particular path length are allowed.Such areas may show up as patches that are lighter or darker than thesurrounding area.

FIG. 19 shows another exemplary pattern 520 of path length differences.In this example, each individual pixel has more than one area of eachpath length. For example, pixels 521, 523 and 525 each have fourregions, which may be four pixel electrode surfaces where two of theregions have one path length and two of the other regions have anotherpath length. The pattern 520 shows an array of 6×4 pixels. This pattern520 may be useful if the pixels are magnified to a large extent, and thesmall differences may be perceptible.

The particular spatial pattern of optical path length difference doesnot necessarily have to be correlated to the spatial pattern of thepixels. The pattern of regions of optical path length difference, forexample, could be randomly or pseudo randomly distributed over theregular array of pixel electrodes. For fringe reduction it is onlyimportant that the regions of optical path length difference be spacedclosely enough on the average to be spatially integrated by the humanvisual system.

Whichever arrangement of path length changes is chosen, it may beadvantageous to build path length changes outside the pixel array in theadjacent peripheral area. This area may be visible to the user of thedisplay, and fringes in this area may also be visible. By constructing apattern of path length variations in the peripheral region, theinvention can be used to suppress the visibility of fringes in theperipheral region. The pattern in the peripheral area could simply be acontinuation of the pattern used over the pixel array, or it could be adifferent pattern that is chosen specifically for the peripheral region.For example, the peripheral region may be used for adhesive deposition,and irregularities in the surface may affect how the adhesive spreads asthe device is assembled. It may be preferable to use a pattern wherepath length differences are arranged in stripes parallel to the adhesiveseal to help prevent flow of the adhesive towards the pixel area.

In the foregoing specification, the invention has been described withreference to specific exemplary embodiments thereof It will be evidentthat various modifications may be made thereto without departing fromthe broader spirit and scope of the invention as set forth in thefollowing claims. The specification and drawings are, accordingly, to beregarded in an illustrative sense rather than a restrictive sense.

1. A display system, comprising: a first electrode; a first substratehaving first and second pluralities of pixel electrodes; and anelectro-optic layer operatively coupled to said first electrode andhaving a first thickness between said first electrode and said firstplurality of pixel electrodes and having a second thickness between saidfirst electrode and said second plurality of pixel electrodes, whereinthe difference between electro-optic thicknesses dispersed betweenclosely spaced pixels is an odd multiple of a one-quarter wavelength oflight which illuminates said first and second pluralities of pixelelectrodes, and said first and second pluralities of pixel electrodesforming an irregular pattern.
 2. A display system according to claim 1wherein said electro-optic layer comprises a liquid crystal materialwhich is disposed between said first electrode and said first substrateand wherein said first electrode is a common counter electrode.
 3. Adisplay system according to claim 2 wherein each of said first andsecond pluralities of pixel electrodes comprises a reflector.
 4. Adisplay system according to claim 1 wherein said display system is areflective micro display.
 5. A display system according to claim 1wherein said first plurality of pixel electrodes and said secondplurality of pixel electrodes are illuminated with the same color light.